To increase the realism of walls, ceilings or other large areas that will be uniform in color, you can make a 2 BSDF material that simulates tiny imperfections/reflective "specs" on the painted wall. To do this, start with a base lambert BSDF, set it to the color of the wall. Avoid RGB values higher than 220 as this will add to the render time and make the walls look unrealistically bright. Please note that when lambert is checked, the ND setting can be ignored.

Add another BSDF, which will represent the reflective specs. The color could be the same as for the lambert BSDF. Set its roughness accordingly; a value of 0 will make the specs perfectly reflective which might not look realistic, so a value of 8-15 is a good starting point. ND could be set high around 3 if you want the specs to be more visible. If you wish for the specs to become visible only at high viewing angles, set the ND to 1.5-2. Add a weightmap to this reflective BSDF that represents the specs. Whiter parts of the weightmap will make this BSDF more visible, darker parts will hide it.

Change the blending mode for this material from Normal to Additive, as we want to simulate reflective specs on top of the base lambert. You can find more info about blending modes in the Maxwell Render manual.

An example of a typical architecture wall material can be found on the MXM Gallery website.